Modern communication networks typically link many different types of mobile and/or stationary communication terminals, such as by way of example, cell phones, computers, and industrial plant equipment, to provide the terminals with an increasing menu of voice, video, and data communication services.
Each communication network operates to transport information from one to another of the terminals in the network using signals containing information relevant to the services that the network provides. In propagating from a source terminal to a destination terminal, the signals generally propagate through a plurality of network nodes that may belong. At each node the signals are received and then, after processing in the node are forwarded toward their destination.
The various services provided by a network and tasks performed at a network's nodes in configuring and/or transporting signals, are often time dependent and nodes that cooperate to configure and transport signals from a source terminal to a destination terminal generally require access to a same common reference time, reference frequency, to provide and/or support an acceptable quality of service (QoS).
For example, a cellular network generally comprises a plurality of base-stations, each of which transmits and receives wireless signals at accurately defined radio frequency (RF) carrier frequencies to and from terminals in a limited geographical area referred to as a “cell” of the network. The wireless signals carry voice and/or data to and from the terminals, which are typically the ubiquitous cell phones. The base-stations are generally connected to each other by a land line network, often referred to as a backhaul network, over which the base-stations communicate with each other to transmit messages between terminals in cells of the network.
Each base-station comprises or has access to its own “local” clock that provides a continuous isochronous train of pulses, referred to as “clock signals” or “clock pulses”, characterized by an accurate and stable pulse repetition frequency, for frequency referencing the base-station operations. A clock as used herein may comprise devices that operate to maintain accuracy and stability of the pulse repetition frequency and/or operate to synchronize the pulses in frequency and time with clock pulses provided by a suitable reference clock. To transport messages and provide network services at an acceptable QoS, all the base-station clocks in the network generally have to operate with reference to substantially a same reference frequency and reference time measured by a Time of Day, (ToD) that coincides substantially with Universal Time Coordinates (UTC).
By way of example, time in cellular networks is usually distributed using a signal comprising a sequence of narrow pulses having a repetition rate accurately maintained at one pulse per second (1-PPS) referenced to UTC. The 1-PPS signal is accompanied by a time code that identifies each 1-PPS pulse by a unique designation, conventionally referred to as a time of day (ToD), which associates a date, month, year, hour, minute and second with the pulse. In a Code Division Multiple Access (CDMA) cellular system, each base-station is assigned a unique time delay offset (TDO) relative to the 1-PPS signals at which it transmits signals. The TDOs enable different base stations to be identified and cell phones to lock on to and receive signals from a base station from which it is intended to receive radio signals. To maintain integrity of the delay offsets and enable the cell phones to properly lock onto intended signals from “correct” base-stations, and successfully negotiate transfer (“handoff”) from one base-station to another as they move from one to the other of the cells in the network, the network base-station clocks are required to be synchronized to a same network time to within an accuracy less than a few microseconds. The spread spectrum coding used in CDMA requires that the base-stations generate RF carrier frequencies that differ from assigned frequencies within error margins that are less than 0.05 ppm (parts per million) to maintain acceptable QoS.
For GSM cellular systems, QoS, as measured for example, by frequency of dropped calls or perceived listening quality, is considered unacceptable for fractional frequency accuracy, Δf/f, of a nominal carrier frequency, “f”, greater than 1 ppm and the ETSI (European Telecommunications Standards Institute) GSM standards requires that base-stations maintain stability of frequency synthesizers and clock generators to within 0.05 ppm.
Usually, a highly accurate reference clock provides a time and a frequency standard for respectively synchronizing time and/or frequency of clocks at nodes, e.g. base-stations, in a network so that the clocks and communication equipment at the nodes operate with reference to substantially a same standard time and a same standard frequency. A reference clock referred to as a Primary Reference Clock (PRC) is expected to provide a measure of frequency accurate to at least 1 part in 1011. A reference clock referred to as Primary Reference Time Clock (PRTC) is expected to provide both a measure of frequency accurate to 1 part in 1011 and time accurate to ±200 ns (nanosecond) relative to Universal Time Coordinated (UTC). A PRC or PRTC may, for example, comprise a GPS radio receiver that receives radio signals provided by GPS satellites and transmits a time reference (e.g. 1-PPS accompanied by a ToD code) and reference clock (e.g. 10 MHz) signals that are responsive to the GPS signals, and which are regulated by a Cesium or Rubidium atomic clock. PRTC time and frequency signals based on radio signals provided by GPS satellites are often used as reference clock signals for synchronizing base-station clocks. In practice, node clocks are synchronized repeatedly and usually at regular time intervals. The process of monitoring and synchronizing a node clock to maintain it synchronized to a reference clock is conventionally referred to as “disciplining” the node clock to the reference.
In some cellular networks, each base-station node comprises a GPS receiver for receiving time reference signals for disciplining its local clock. In many cellular networks, the core network is responsible for distributing time and frequency information, hereinafter “timing information”, to and from nodes in the network for disciplining their local clocks. The timing information is generated responsive to an accurate reference time and/or frequency provided by a reference clock at a node of the core network or by a reference clock to which the core network has access. A reference clock in such networks, which provides a time and/or a frequency reference for providing timing information for disciplining other clocks in the network, may also be referred to as a “master clock”. A clock in the network that is disciplined responsive to the timing information may also be referred to as a “slave clock”.
Various methods are used by cellular networks for distributing timing information, also referred to as “distributing time”, over a core network from master clocks to slave clocks for disciplining the slave clocks. The methods are substantially different for synchronous and asynchronous communication networks. Synchronous and asynchronous communication networks transport information using procedures that are inherently very different, and conventional methods of distributing time and/or frequency to, and disciplining, slave clocks at nodes of synchronous networks are generally not applicable to asynchronous network.
In a synchronous communication networks, such as Synchronous Digital Hierarchy (SDH) or Synchronous Optical Networks, (SONET), communication between nodes is conducted over fixed paths having substantially constant latencies provided by physical links that connect the nodes. Signals are transmitted between first and second nodes in the network in well ordered sequential series of symbols having a substantially constant repetition frequency. The repetition frequency is rigorously maintained by apparatus at each node along the fixed path between the first and second nodes to within a small deviation from a predetermined standard network frequency.
All nodes in a path in the network typically have reference to a primary reference clock (PRC), from which they discipline their own local clocks. The PRC periodically sends a series of symbols compliant with the network protocols to a first node in the path, which locks onto the frequency of the symbols and adjusts its local clock to match the frequency of the received symbols. Generally, the local clocks implement a Phase Locked Loop (PLL) circuit comprising a phase error detector, a low pass filter, and a Frequency Synthesis Device (FSD), that cooperate to lock the local clock frequency to the frequency of the PRC. After the first node in the path synchronizes its clock, it transmits the series of symbols downstream along the path to a next node, which similarly adjusts the frequency of its clock and proceeds to transmit the symbol sequence downstream to a next node. The process is repeated until all the clocks in all the nodes along the path are synchronized.
In an asynchronous network, such as an Ethernet, IP, or MPLS, packet switched network (PSN), information is not transported over fixed paths between terminals and/or nodes in strictly sequential trains of symbols characterized by a constant symbol repetition rate. Information in a message transported by a PSN network is configured in packets that may travel between the same two nodes and/or terminals in the network along different paths and may experience different transit times, referred to as packet delays (PDs), in propagating between the nodes and/or terminals. For example, the availability of a plurality of alternate paths in a PSN for transporting packets between two nodes often generates a difference, referred to as a delay asymmetry, in transit time of a packet propagating from a first to a second of the nodes compared to transit time of a packet propagating from the second to the first of the nodes. Whether packets travel the same or different paths through the PSN they are generally subject to variation in packet delay (PDV) as a result for example of varying queuing delay along that path that affects their transit times, and typically introduces statistical variations in PD between two points in the network. As a result, the relatively straightforward, direct manner in which frequency is distributed in a synchronous communication network does not apply for a PSN.
To discipline a slave clock in a PSN, a master clock at a node or terminal of the PSN periodically exchanges a sequence, conventionally referred to as a “transaction”, of “timing messages” with the slave clock. The timing messages comprise timing information configured in data packets, hereinafter referred to as “timing packets”, compliant with the protocols of the PSN network. The timing packets comprise timing information, conventionally referred to as “timestamps”, which the slave clock records, and which define times at which the timing packets egress and/or ingress the master clock and/or the slave clock. Upon completion of a transaction, the slave clock has a record comprising a set of timestamps that it uses to synchronize itself to the master clock.
Delay asymmetry and PDV inherent in a PSN network, such as a PSN backhaul network of a cellular network, make disciplining slave clocks of the network responsive to timestamps acquired in transactions, generally more complicated and error prone than disciplining clocks of a synchronous core network. Furthermore, as a number of clocks in a PSN increases, the difficulty, expense, and bandwidth overhead incurred to discipline the network clocks increases.
Whereas maintaining synchronization for PSNs is inheritably more difficult and error prone than for synchronous networks, PSNs provide substantially more efficient and flexible use of bandwidth than synchronous networks. PSNs therefore are replacing legacy SONET/SDH synchronous networks used in cellular backhaul networks and are configured and operated to support required frequency and time accuracies demanded of base-station nodes of backhaul networks. However, to meet QoS standards of anticipated increases in cellular traffic and new cellular network services, PSN networks have to anticipate providing improved monitoring of accuracy with which they distribute frequency and time to network nodes.